1. Field of Invention
The present invention relates to a soft-start circuit and a method thereof, and more particularly to a circuit capable of ensuring a smooth soft-start and preventing surge current during operation, and a method thereof.
2. Related Art
A switching regulator having a high voltage conversion efficiency is often used to convert a current with a significant voltage difference and large load. However, an excessive rush current or overshoot of output voltage often occurs soon after the switching regulator is powered on, and results in damage to the circuit. Therefore, when the switching regulator is powered on, a soft-start device is required to make the supply voltage rise slowly. In addition to the switching regulator, the soft-start device can be applied to many other circuits, as long as these circuits require the input voltage to rise slowly to avoid excessive rush current or overshoot of output voltage being generated soon after they are powered on.
FIG. 1A is a schematic view of a soft-start device in the prior art. Referring to FIG. 1A, the soft-start device in the prior art includes a current source A10, a capacitor A20, a switch A30, a reference voltage A40, and a fixed voltage source A50.
The soft-start device is an open loop operation at first, that is, the switch A30 is open. The current source A10 charges the capacitor A20. During the charging process, the voltage of the capacitor A20 rises slowly to generate a ramp voltage. The reference voltage A40 rises slowly with the ramp voltage, so as to ensure a soft-start. At this time, while the ramp voltage is close to the voltage of the fixed voltage source A50 (which may be a bandgap voltage), the soft-start mechanism must finish in order for the system to resume normal operation. In detail, the switch A30 is closed and returned to a closed loop, such that the reference voltage A40 outputs the voltage of the fixed voltage source A50.
However, the time point at which to switch the switch A30 is difficult to control, so a discontinuous surge often occurs, making the system unstable and causing errors. FIG. 1B is a schematic waveform diagram (I) of the soft-start device in the prior art. As shown in FIG. 1B, if the switch A30 is switched too late, the reference voltage (Vref) A40 may exceed the bandgap voltage (Vbg), and form an upward surge before returning to the bandgap voltage Vbg. FIG. 1C is a schematic waveform diagram (II) of the soft-start device in the prior art. As shown in FIG. 1C, if the switch A30 is switched too early, the reference voltage Vref A40 may not reach the bandgap voltage Vbg, so that a gap exists between the reference voltage A40 and the bandgap voltage.
Therefore, the problems in the prior art (derived from the discontinuous voltage waveform when the switch of the soft-start device is turned on/off, or derived from the current surge during the operation of the switching regulator), must be solved.